Can The Degree of Hydronephrosis on Ultrasound Predict Kidney Stone Size?

Background

Symptomatic renal colic is a common complaint presenting to the emergency department (ED), with a rate of 126 to 226 per 100,000 ED visits [1]. In the ED, CT is frequently used to make the definitive diagnosis as it allows for determination of stone size and location, degree of hydronephrosis, and evaluation of other pathology that may mimic renal colic. However this is a particularly worrisome approach in patients with recurrent ureteral stones who have been exposed to numerous previous CT imaging studies. Previous data has shown that emergency physician performed ultrasound is accurate at identifying hydronephrosis, which in combination with hematuria, is sufficient for the diagnosis of renal colic [2,3]. Furthermore, an ultrasound first approach has been shown to be safe and reasonable as an initial evaluation for suspected renal colic [4]. What ultrasound does not tell us about renal colic is the size of the ureteral stone, which can be useful in determining the need for immediate intervention versus medical management. The following study seeks to determine if the degree of hydronephrosis seen on ultrasound performed by emergency physicians, can be predictive of ureteral stone size.

Each focused renal ultrasound classified the degree of hydronephrosis as none, mild, moderate, or severe and this was compared to the ureteral stone size on noncontrast CT

Definitions:

Mild hydronephrosis was defined as enlargement of the calices withpreservation of the renal papillae

Moderate hydronephrosis was defined as rounding of the calices with obliteration of therenal papillae

Severe hydronephrosis was defined as caliceal ballooning with cortical thinning

Ureteral stone size was stratified into 2 groups, those 5mm or smaller and those larger than 5 mm, based on the likelihood of successfully spontaneous stone passage

Results

Increasing degree of hydronephrosis seen on focused ultrasound was associated with an increasing proportion of ureteral calculi larger than 5 mm. 113 (87.6%) patients with less severe hydronephrosis (none or mild) had ureteral calculi 5 mm or smaller. Of the remaining 16 (12.4%) patients with less severe hydronephrosis, none of these patients had ureteral stones larger than 10 mm. There was good interobserver agreement between the degree of hydronephrosis as determined by the performing emergency physician and the quality assurance review (k = 0.847).

Strengths & Limitations

Strengths

Majority of ultrasound examinations performed by ED physicians making this applicable to point-of-care ultrasound

This study only enrolled patients who both a focused renal ultrasound and confirmed ureterolithiasis on noncontrast CT; this would have missed patients who only had either a focused renal ultrasound or noncontrast CT alone (selection bias)

No patient centered outcomes data

Authors Conclusion

"In conclusion, our results demonstrate a relationship between the degree of hydronephrosis as determined by emergency physicians on focused emergency ultrasound and ureteral calculi size; patients with less severe hydronephrosis were less likely to have larger ureteral calculi. This suggests that ultrasound can help identify many, but not all, patients who are at lower risk for having larger ureteral calculi.”

Our Conclusion

This paper identifies a correlation between the degree of hydronephrosis on ultrasound and ureteral stone size seen on noncontrast CT. Essentially, patients with minimal or no hydronephrosis are very unlikely to have have a large (>5 mm) ureteral stone. Unfortunately, focused ultrasound is not perfect, and in this study ~12.4% of patients with minimal or no hydronephrosis still had a large ureteral stone. What I found reassuring was that in this group, none of the patients had a ureteral stone > 10 mm, which at most institutions is the cut off for allowing a trial of passage. Even dissecting the data further, of the patients with moderate hydronephrosis, only 2 out of 43 (4.6%) patients had a stone > 10 mm.

This study suggests that focused renal ultrasound can be used to screen patients with suspected renal colic and potentially avoid an unnecessary CT scan. As with any focused ultrasound, the decision to obtain a CT should not be based solely the degree of hydronephrosis but also in conjunction with the clinical history, physical exam and other pertinent factors (previous ureteral stone, previous need for stone intervention, other concerning diagnoses on differential, pain control, institutional culture, urinalysis, etc).

The Bottom Line

Ultrasound can be used to identify many, but not all, patients who are at lower risk for having larger ureteral calculi.

Authors

This post was written by Michael Macias, MD, Ultrasound Fellow at UCSD.